Plug-ins allow Firefox to use third-party software to display multimedia and other programs in the web browser, and themes change the appearance of Firefox. Extensions, on the other hand, provide Firefox with new features, or take existing features and extend them in new ways.
There are several ways you can acquire Firefox extensions, but the official way is by selecting Tools -> Extensions to bring up Firefox's Extension Manager. When the Extension Manager window opens, you will see a list of currently installed extensions (of course, when you first install Firefox you don't have any extensions installed, so this part of the window is blank). At the bottom of the window are three buttons Uninstall, Update, and Options and a Get More Extensions link.
If you select an already installed extension and press Uninstall, the extension will be fully removed once you restart Firefox. To see if an extension has an update available, select it and then press Update. Firefox will check, and if there is an update, you will be prompted to install it. Finally, if an extension has preferences available, the Options button will be available to press in order to access those preferences; otherwise, the button will be grayed out and will not work.
The Extension Manager actually allows you do more than just uninstall, update, or view options. Right-click on a selected extension, and a contextual menu opens with several other possibilities.
Three of the choices on this menu duplicate the buttons at the bottom of the Extension Manager window: Options, Uninstall, and Update. Most of them, however, are new. Visit Home Page takes you to the home page of the extension, which can be really useful if you want to gather more information about it. The About choice opens a small window that lets you know who worked on the extension, what the current version number is, and what the extension is designed to do.
Disable is a nice addition to Uninstall and Update. What if you just want to turn off an extension temporarily? Maybe it's causing a problem on a web site, or maybe you think it might be causing a problem with Firefox. Disable the extension and see what happens.
The last three options help you position the extensions in the order you'd like. By default, extensions are listed in the order in which you added them, with the oldest at the top. To reorder your extensions, right-click on an extension and choose Move to Top, Move Up, or Move Down. If you have a lot of extensions, this can grow tedious, and most people probably won't care what order their extensions are in. For those of you who like to have everything just so, however, I'm sure you'll appreciate the ability to reorder your list of extensions.
Firefox gives you a lot of power with extensions, but thankfully, a couple of features greatly reduce the likelihood that an extension will cause a security or stability problem. First, Firefox comes with a built-in whitelist of sites that can install extensions, including update.mozilla.org by default. If you find a cool extension at a site that isn't on the whitelist and try to install it, Firefox shows an alert at the top of browser.
Firefox does everything it can to ensure that extensions don't damage your installation of the web browser. Still, if you want to install multiple extensions, we recommend installing them one at a time and restarting the browser and testing each extension as you go, and then repeating the process with the next extension. Yes, it's entirely possible to install 10 extensions at a time before restarting Firefox, and things may well work just fine, but if you do have a problem, it'll be far more difficult to diagnose the source. Better to take it slow and install them one at a time, immediately removing any that cause problems.